An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer(s) is a film of or based mainly on organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes in some cases. Generally, for example, at least one of these electrodes is transparent. OLEDs sometimes are used in television screens; computer monitors; small or portable system screens such as those found on mobile phones and PDAs; and/or the like. OLEDs may also sometimes be used in light sources for space illumination and in large-area light-emitting elements. OLED devices are described, for example, U.S. Pat. Nos. 7,663,311; 7,663,312; 7,662,663; 7,659,661; 7,629,741; 7,601,436, 2011/0193477, and 2009/0295283, the entire contents of all of which are hereby incorporated herein by reference.
A typical OLED comprises at least two organic layers—e.g., electron and hole transport layers—that are embedded between two electrodes. One electrode typically is made of a reflective metal. The other electrode typically is a transparent conductive layer supported by a glass substrate. The one electrode generally is the cathode, and the other electrode generally is the anode. Indium tin oxide (ITO) often is used at the front portion of the OLED as the anode.
FIG. 1 is an example cross-sectional view of a typical OLED. The OLED includes glass substrate 102, transparent conductive anode layer 104, organic layer 100, cathode layer 110 and cover glass 112. The organic light emission layer 100 emits light, and light is generated by processes known from conventional OLEDs when electrons and holes injected into the organic layer 100 from different sides recombine. The organic layer 100 may include multiple layers. For example, in certain example instances the organic layer 100 may include a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer. An example shown in FIG. 1 illustrates the organic semiconductor layer 100 including a hole transport layer (HTL), and electron transport layer (ETL), and an emitting layer, where the ETL and emitting layer may or may not be present in one layer.
When a voltage is applied to the electrodes 104 and 110, the charges start moving in the device under the influence of the electric field. Electrons leave the cathode, and holes move from the anode in opposite direction. For example, the recombination of these charges leads to the creation of photons with frequencies given by an energy gap between LUMO and HOMO levels of the emitting molecules, so that the electrical power applied to the electrodes is transformed into light. Different materials and/or dopants may be used to generate different colors, with the colors being combinable to achieve yet additional colors.
This disclosure relates to an improved electrode on the light-emitting side of the organic layer, for use in an OLED device or the like. For example, referring to the OLED in FIG. 1, this disclosure relates to an improved electrode 104 on the light emitting side of the organic layer 100 and a method of making the same.
In certain example embodiments of this invention, the electrode is made by depositing first and second successive transparent conductive oxide (TCO) layers of the same material (e.g., indium-tin-oxide; a/k/a ITO) with different respective stoichiometries. The first and second TCO layers are deposited (directly or indirectly) on a substrate (e.g., glass substrate), and may be deposited for example via sputtering at approximately room temperature. Sputter deposition without intentional heating of the substrate offers certain cost advantages due, primarily, to the lower capital, lower maintenance and higher deposition rates. At the same time, additional post-deposition heating of the TCO is usually used to optimize its optical and electrical performance. High temperatures of post-deposition heating allow better TCO properties. At ˜350-500 C, however, the room temperature deposited ITO abruptly loses its conductivity when heated in air, unless its stoichiometry is substantially sub-oxided (a lack of oxygen in the TCO composition). The properties of the sub-oxided ITO, however, are not optimum after the heat treatment. To achieve the benefits of high-temperature post-deposition heat treatment while avoiding the conductivity loss, example embodiments of this invention introduce at least a double-layer TCO, where the second layer serves as a protection to the first layer optimized for the best TCO performance after the heat treatment. The first TCO layer is located between the substrate and the second TCO layer, and the first TCO layer is more oxided as deposited than is the second TCO layer as deposited, while both are of the same material or substantially the same material. In certain example embodiments, as deposited, the first TCO layer contains at least about 5% more oxygen, more preferably at least about 10% more oxygen, even more preferably at least about 20% more oxygen, and most preferable at least about 30% more oxygen, than does the second TCO layer. The TCO layers of the electrode on the substrate may then be heat treated (HT) at high temperature(s) in order to (a) activate the electrode for desired electrical and/or optical properties, (b) increase the work function (WF) of the electrode, and/or (c) increase visible transmission of the electrode.
In certain example embodiments of this invention, there is provided a method of making an electrode for use in an organic light emitting diode (OLED) device, the method comprising: sputter-depositing a first layer comprising indium tin oxide (ITO) on a glass substrate; sputter-depositing a second layer comprising ITO on the glass substrate over and directly contacting the first layer comprising ITO to form an electrode structure, so that the first layer comprising ITO is located between at least the substrate and the second layer comprising ITO in the electrode structure, wherein as sputter-deposited the first layer comprising ITO is more oxided than is the second layer comprising ITO; and heat treating the electrode structure including the glass substrate and the first and second layers comprising ITO at temperature(s) of at least about 400 degrees C. in order to increase work function of ITO in the electrode structure and increase visible transmission of the electrode structure. As sputter-deposited, the first layer comprising ITO may contain at least 5% more oxygen, more preferably at least 10% more oxygen, still more preferably at least 20% more oxygen, and even more preferably at least 30% more oxygen (by mol %), than the second layer comprising ITO.
In certain embodiments of this invention, there is provided a method of making an electrode for use in an organic light emitting diode (OLED) device, the method comprising: sputter-depositing a first layer comprising metal oxide on a substrate; sputter-depositing a second layer comprising metal oxide on the substrate over and directly contacting the first layer comprising metal oxide to form an electrode structure, wherein the same metal oxide is in both the first and second layers comprising metal oxide, but as sputter-deposited the first layer comprising metal oxide is more oxided than is the second layer comprising metal oxide; and heat treating the electrode structure including the substrate and the first and second layers comprising metal oxide at temperature(s) of at least about 400 or 500 degrees C. in order to increase work function of ITO in the electrode structure and increase visible transmission of the electrode structure. The substrate may comprise glass or quartz, and the metal oxide may be or comprise ITO, or alternatively the metal oxide may be another suitable metal oxide.
In certain embodiments of this invention, there is provided an organic light emitting diode (OLED) comprising: a transparent conductive electrode structure comprising first and second layers comprising the same metal oxide (e.g., ITO) on a substrate (e.g., glass or quartz substrate), the first layer comprising the metal oxide being located between the substrate and the second layer comprising the metal oxide, and wherein the second layer comprising the metal oxide directly contacts the first layer comprising the metal oxide; wherein the same metal oxide is in both the first and second layers comprising the metal oxide, and wherein the first layer comprising the metal oxide is more oxided than is the second layer comprising the metal oxide; and an organic light emitting layer located between said transparent conductive electrode structure and another electrode, and wherein said transparent conductive electrode structure is on a light emitting side of the organic light emitting layer.
In certain embodiments of this invention, there is provided an organic light emitting diode (OLED) comprising: an organic light emitting layer located between a transparent conductive electrode structure and another electrode, and wherein said transparent conductive electrode structure is on a light emitting side of the organic light emitting layer; and wherein said transparent conductive electrode structure includes a film comprising a metal oxide (e.g., ITO), wherein said film comprising the metal oxide includes a first portion and a second portion, the first portion being more oxided than the second portion and being farther from the organic light emitting layer than the second portion. In certain example embodiments, the first portion may contain at least 5% (more preferably at least 10%, even more preferably at least 20% or 30%) more oxygen than the second portion. The film comprising the metal oxide may be oxidation graded, continuously or discontinuously, so as to be more oxided farther from the organic light emitting layer.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.